In order to achieve greater transmission rate, it has been proposed to use a cooperative beamforming, where a plurality of antennas from different cells are used to create at least one spatial stream towards a user terminal. In connection with the beamforming and MIMO techniques, a precoding is applied on the antennas by applying complex coefficients on the antennas, so that the gain and/or the phase of the transmitting antennas are modified. It may be nevertheless crucial to signal to the user terminal some information about the beamforming (e.g. so that the user terminal can derive a suitable phase reference for receiving the signal).
This can be done by means of predetermined codebooks, being a set of predetermined coefficients. An index of the selected codebook may be signalled for instance to the receiving station, so that a corresponding set of receiving coefficients is applied. Moreover, in order to provide feedback to the transmitting stations, the user terminal needs to indicate information about the channel, for example a preferred precoding vector. It is thus required that all this signalling is implemented in a clever way so that it does not create too much overhead, but it is still reliable and effective enough.
In systems such as UMTS and LTE, multi-antenna transmission/reception techniques variously described as, MIMO, precoding or beamforming are supported for transmissions from a single cell to a mobile terminal. Precoding codebooks are defined, which enable the User Equipment (UE) to report a preferred precoding index for downlink transmission.
The same codebook may be used to signal to a user equipment the precoding vector or matrix which is actually applied in the downlink by a base station. This may be viewed as a way of describing the channel coefficients. This enables the user equipment to derive an appropriate phase/amplitude reference signal from common pilot symbols for demodulation of downlink transmissions. Alternatively the pilot symbols may be precoded to form dedicated reference symbols and used directly as receiver phase reference.
Typically, the specification for systems like LTE makes use of the term “antenna port”, which is effectively a virtual antenna which may be derived by a linear combination of signals from one or more physical antennas. For convenience, we use the term “antenna”, but this could also be understood as “antenna port”
Multi-antenna techniques such as beamforming, using antennas from multiple cells or multiple sites, could be of interest in LTE. However, there is a need of specifying how codebooks should be defined for such cases so that the signalling may be simple but still effective.